Introduction to Elementary Particles E-Book

David Griffiths

• Herausgeber: Wiley-VCH GmbH
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch
• Veröffentlichungsjahr: 2020

Seitenzahl: 820

Beschreibung

Die Elementarteilchenphysik ist auf der ganzen Welt ein fester Bestandteil im Curriculum des Physikstudiums. Umso wichtiger ist es daher, dass auf diesem Gebiet bereits in den ersten Semestern ein solides Wissensfundament gelegt wird - nicht zuletzt als Vorbereitung auf die Themenbereiche Hochenergie- oder Kernphysik. In diesen Band ist die gesamte Lehrerfahrung von David Griffiths eingeflossen - eine begehrte "Ware", die in der Neuauflage nun auch ein Lösungsmanual präsentiert, das die zahlreichen Aufgaben und Fragen der Kapitelenden aufnimmt. Der Autor versteht es, sich den Themen in einer lebendigen Sprache zu nähern, die jedoch im Hinblick auf Präzision keine Kompromisse eingeht. So eröffnet der Band den Zugang zu den Theorien ebenso wie zu Modellen und Rechenoperationen. Das Werk wird von vielen Lehrenden empfohlen und kann bereits jetzt als Klassiker innerhalb der einführenden Werke zur Elementarteilchenphysik bezeichnet werden.

Leseprobe

David Griffiths

Introduction to Elementary Particles

Second, Revised Edition

The Author

David GriffithsReed College Portland, OR USA

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ISBN: 978-3-527-40601-2

Preface to the First Edition

This introduction to the theory of elementary particles is intended primarily for advanced undergraduates who are majoring in physics. Most of my colleagues consider this subject inappropriate for such an audience – mathematically too sophisticated, phenomenologically too cluttered, insecure in its foundations, and uncertain in its future. Ten years ago I would have agreed. But in the last decade the dust has settled to an astonishing degree, and it is fair to say that elementary particle physics has come of age. Although we obviously have much more to learn, there now exists a coherent and unified theoretical structure that is simply too exciting and important to save for graduate school or to serve up in diluted qualitative form as a subunit of modern physics. I believe the time has come to integrate elementary particle physics into the standard undergraduate curriculum.

Unfortunately, the research literature in this field is clearly inaccessible to undergraduates, and although there are now several excellent graduate texts, these call for a strong preparation in advanced quantum mechanics, if not quantum field theory. At the other extreme, there are many fine popular books and a number of outstanding Scientific American articles. But very little has been written specifically for the undergraduate. This book is an effort to fill that need. It grew out of a one-semester elementary particles course I have taught from time to time at Reed College. The students typically had under their belts a semester of electromagnetism (at the level of Lorrain and Corson), a semester of quantum mechanics (at the level of Park), and a fairly strong background in special relativity.

In addition to its principal audience, I hope this book will be of use to beginning graduate students, either as a primary text, or as preparation for amore sophisticated treatment. With this in mind, and in the interest of greater completeness and flexibility, I have included more material here than one can comfortably cover in a single semester. (In my own courses I ask the students to read Chapters 1 and 2 on their own, and begin the lectures with Chapter 3. I skip Chapter 5 altogether, concentrate on Chapters 6 and 7, discuss the first two sections of Chapter 8, and then jump to Chapter 10.) To assist the reader (and the teacher) I begin each chapter with a brief indication of its purpose and content, its prerequisites, and its role in what follows.

This book was written while I was on sabbatical at the Stanford Linear Accelerator Center, and I would like to thank Professor Sidney Drell and the other members of the Theory Group for their hospitality.

DAVID GRIFFITHS

1986

Preface to the Second Edition

It is 20 years since the first edition of this book was published, and it is both gratifying and distressing to reflect that it remains, for the most part, reasonably up-to-date. There are, to be sure, some gross lacunae – the existence of the top quark had not been confirmed back then, and neutrinos were generally assumed (for no very good reason) to be massless. But the Standard Model, which is, in essence, the subject of the book, has proved to be astonishingly robust. This is a tribute to the theory, and at the same time an indictment of our collective imagination. I don’t think there has been a comparable period in the history of elementary particle physics in which so little of a truly revolutionary nature has occurred. What about neutrino oscillations? Indeed: a fantastic story (I have added a chapter on the subject); and yet, this extraordinary phenomenon fits so comfortably into the Standard Model that one might almost say, in retrospect (of course), that it would have been more surprising if it had not been so. How about supersymmetry and string theory? Yes, but these must for the moment be regarded as speculations (I have added a chapter on contemporary theoretical developments). As far as solid experimental confirmation goes, the Standard Model (with neutrino masses and mixing) still rules.

In addition to the two new chapters already mentioned, I have brought the history up-to-date in Chapter 1, shortened Chapter 5, provided (I hope) a more compelling introduction to the Golden Rule in Chapter 6, and eliminated most of the old Chapter 8 on electromagnetic form factors and scaling (this was crucially important in interpreting the deep inelastic scattering experiments that put the quark model on a secure footing, but no one today doubts the existence of quarks, and the technical details are no longer so essential). What remains of Chapter 8 is now combined with the old Chapter 9 to make a new chapter on hadrons. Finally, I have prepared a complete solution manual (available free from the publisher, though only – I regret – to course instructors). Beyond this the changes are relatively minor.

Many people have sent me suggestions and corrections, or patiently answered my questions. I cannot thank everyone, but I would like to acknowledge some of those who were especially helpful: Guy Blaylock (UMass), John Boersma (Rochester), Carola Chinellato (Brazil), Eugene Commins (Berkeley), Mimi Gerstell (Caltech), Nahmin Horwitz (Syracuse), Richard Kass (Ohio State), Janis McKenna (UBC), Jim Napolitano (RPI), Nic Nigro (Seattle), John Norbury (UW-Milwaukee), Jason Quinn (Notre Dame), Aaron Roodman (SLAC), Natthi Sharma (Eastern Michigan), Steve Wasserbeach (Haverford), and above all Pat Burchat (Stanford).

Part of this work was carried out while I was on sabbatical, at Stanford and SLAC, and I especially thank Patricia Burchat and Michael Peskin for making this possible.

DAVID GRIFFITHS

2008

References

1 For a comprehensive bibliography see Chao, Q. W. (

2006

)

Amer

ican Journal of Physics

,

74

, 855.

2 Smith, C. L. (July

2000

)

Scientific American

, 71; (a) Lederman, L.(

2007

)

Nature

,

448

, 310.

3 The current reference is Yao, W.-M.

et al

. (

2006

)

Journal of Physics

,

G 33

,

1. But because you will want to use the most recent versions,I will simply refer to them as

Particle Physics Booklet

and

Review of Particle Physics

, appending the year when it is relevant.

Notes

*

  In macroscopic terms the amount of energy involved is not that great – after all, 1 TeV (10

12

eV) is only 10

−7

Joules; the problem is how to

deliver

that energy to a particle. No law of physics prevents you from doing so, but nobody has yet figured out a way to do it without gigantic (and expensive) machinery.

1Historical Introduction to the Elementary Particles

This chapter is a kind of ‘folk history’ of elementary particle physics. Its purpose is to provide a sense of how the various particles were first discovered, and how they fit into the overall scheme of things. Along the way some of the fundamental ideas that dominate elementary particle theory are explained. This material should be read quickly, as background to the rest of the book. (As history, the picture presented here is certainly misleading, for it sticks closely to the main track, ignoring the false starts and blind alleys that accompany the development of any science. That’s why I call it ‘folk’ history – it’s the way particle physicists like to remember the subject – a succession of brilliant insights and heroic triumphs unmarred by foolish mistakes, confusion, and frustration. It wasn’t really quite so easy.)

1.1 The Classical Era (1897–1932)

It is a little artificial to pinpoint such things, but I’d say that elementary particle physics was born in 1897, with J. J. Thomson’s discovery of the electron [1]. (It is fashionable to carry the story all the way back to Democritus and the Greek atomists, but apart from a few suggestive words their metaphysical speculations have nothing in common with modern science, and although they may be of modest antiquarian interest, their genuine relevance is negligible.) Thomson knew that cathode rays emitted by a hot filament could be deflected by a magnet. This suggested that they carried electric charge; in fact, the direction of the curvature required that the charge be negative. It seemed, therefore, that these were not rays at all, but rather streams of particles. By passing the beam through crossed electric and magnetic fields, and adjusting the field strength until the net deflection was zero, Thomson was able to determine the velocity of the particles (about a tenth the speed of light) as well as their charge-to-mass ratio (Problem 1.1). This ratio turned out to be enormously greater than for any known ion, indicating either that the charge was extremely large or the mass was very small. Indirect evidence pointed to the second conclusion. Thomson called the particles corpuscles. Back in 1891, George Johnstone Stoney had introduced the term ‘electron’ for the fundamental unit of charge; later, that name was taken over for the particles themselves.

Thomson correctly surmised that these electrons were essential constituents of atoms; however, since atoms as a whole are electrically neutral and very much heavier than electrons, there immediately arose the problem of how the compensating plus charge – and the bulk of the mass – is distributed within an atom. Thomson himself imagined that the electrons were suspended in a heavy, positively charged paste, like (as he put it) the plums in a pudding. But Thomson’s model was decisively repudiated by Rutherford’s famous scattering experiment, which showed that the positive charge, and most of the mass, was concentrated in a tiny core, or nucleus, at the center of the atom. Rutherford demonstrated this by firing a beam of α particles (ionized helium atoms) into a thin sheet of gold foil (Figure 1.1). Had the gold atoms consisted of rather diffuse spheres, as Thomson supposed, then all of the α particles should have been deflected a bit, but none would have been deflected much – any more than a bullet is deflected much when it passes, say, through a bag of sawdust. What in fact occurred was that most of the α particles passed through the gold completely undisturbed, but a few of them bounced off at wild angles. Rutherford’s conclusion was that the α particles had